1. Field of the Invention
This invention relates in general to telecommunication between earth stations and satellites. The invention is directed to both a telecommunication apparatus and method.
2. Description of the Related Art
It is known to communicate among various earth stations via satellites in orbit around the earth. Various configurations of earth stations allow them to interface with telephone systems, facsimile machines, telex, electronic mail, etc. Generally, such telecommunication arrangements are configured to transmit and receive on multiple channels over a wide band of frequencies.
FIG. 7 (PRIOR ART) shows a known arrangement of an earth station used for telecommunication via a satellite. The apparatus includes a transmitter 901 providing a transmitting signal which is modulated by a modulating signal and an oscillator 902 for generating a constant frequency signal. The transmitting signal is supplied to an up-converter 905 through a coaxial cable 903 and a multiplexer 904. The signal from oscillator 902 is also supplied to a multiplier 906 through coaxial cable 903 and multiplexer 904. A multiplier 906 multiples the frequency of the signal from oscillator 902 to produce a local oscillating signal. An up-converter 905 generates a radio frequency (RF) signal corresponding to the sum of frequencies of the transmitting signal and the local oscillating signal. The RF signal is supplied to a power amplifier 907 for producing and amplified signal to be transmitted from an antenna 909 through a duplexer 908.
Also, antenna 909 receives RF signals from satellites to supply received signals to a low noise amplifier 910 through duplexer 908. A down-converter 911 generates an intermediate frequency (IF) signal corresponding to the difference between both frequencies of the amplified signal from amplifier 910 and the local oscillating signal. The IF signal is supplied to a receiver 912 for demodulating the received signals through multiplexer 904 and coaxial cable 903. A frequency band of transmitted signals from antenna 909 is separated by a defined frequency from a frequency band of the RF signals from satellites.
RF signals are transmitted and received by antenna 909 at different times. Transmitting and receiving can occur on multiple channels simultaneously. Thus, a transmission system for transmitting IF signals to receiver 912 is arranged to have wideband characteristics because the IF signals have various frequencies corresponding to the various communication channels. If the overall power level of the communication arrangement is to remain constant, the power of a signal in one channel cannot rise significantly. Therefore, because power levels of the IF signals are set so as to be relatively low, received signals are easily disturbed by external noise and by internal noise of the apparatus including undesired spurious signals.
It is difficult to frequency multiplex for transmission via a communication cable because the IF signals which are converted by down-converter 911 are distributed over a wide band width.